Thomas Malthus‘ 1766-1834 famous book of (1798 revised 1803, see: 1826) An Essay on the Principle of Population as it Affects the Future Improvement of Society, was being read by both Charles Darwin and Alfred Russel Wallace when they had their independent realizations on the origin of species. Malthus is rated #77 in the book The 100: A ranking of the most influential persons in history by Michael H. Hart.
But, he didn’t rate a mention in Charles Murray’s book Human Accomplishment.
Principle of Population III.I.1
. . . But the truth is, that, if the view of the argument given in this essay be just, the difficulty, so far from being remote, is imminent and immediate. At every period during the progress of cultivation, from the present moment to the time when the whole earth was become like a garden, the distress for want of food would be constantly pressing on all mankind, if they were equal. Though the produce of the earth would be increasing every year, population would have the power of increasing much faster, and this superior power must necessarily be checked by the periodical or constant action of moral restraint, vice, or misery.
A key word in this paragraph is, “if they were equal.” That is if all the food of the world were distributed with absolute equality then the entire population would grow or die at the same time. But clearly as Malthus implied, all food and other resources are never distributed equally and thus they thrive or die because of some inequality. Both Darwin and Wallace were struck by this idea and pondered what those inequalities might be and how that affected the survival of the individual and the species. If the individual survives it has the opportunity to reproduce but if it fails to survive it has no opportunity to reproduce. That is obvious enough but what is it that gives an individual that opportunity? There are many things and that is perhaps what confused the issue for so long because the potential causes of survivability are nearly infinite even within a given species. And with a vast number of species, each with their individual needs and multiplicity of environments, the trait which gives improved survivability is a near infinity times an infinity times an infinity. That complexity of potential survival traits clouds the issue of how to think about it and state one’s thoughts clearly.
It was on that point which Malthus cut through to the essence of the problem and on which both Darwin and Wallace had their flash of inspiration. That a heritable trait which conferred an advantage upon an individual to survive, no matter how slight that advantage might be, would become incorporated into the species’s batch of available traits for its members.
There are some things which must be fulfilled for an individual or a species to survive and one of the most basic is an energy source. On a short term level this energy is in the form of food of which there must be some minimal level but there are other forms of energy almost as vital such as warmth from the sun. Or other forms of behavior which can ease the need for food to warm the individual such as making underground homes to protect one from the weather, too hot, too cold, too wet, too dry. Each of those things can be coped with to some degree by the individual’s biological adaptations but these adaptations generally require energy and that energy comes from food.
Sometimes a species can hibernate to conserve energy, or sometimes they can make seeds which will last many years and revivify the species when environmental conditions are good. Ultimately it always comes back to the energy supplied by food and an environment benign enough to support the species.
Malthus’s idea that was taken up by Darwin and Wallace was that populations will increase as rapidly as possible, generally said to be logarithmic but that doesn’t really matter as unchecked arithmetic increase will eventually achieve the same results. But he said food supply could only increase at the rate at which food could be cultivated. There is no limit to how large a population might conceivably grow if given absolute freedom to reproduce but there is a limit to how much food the Earth can produce. Since Malthus’s time humans have managed to find many new ways to create food for themselves but the fact remains the Earth is limited in size and at some time a limit will be reached. If there is no check on a population it will grow indefinitely but if it grows indefinitely there will come a time when it seasonally equals or exceeds the food supply. Then there will be poverty, famine and misery and a sudden decline in population. In the special case of humans they will be forced to work for the absolute minimum wage that will sustain their life. This is sometimes called, the iron law of wages. Even Aristotle two millennia before had written, ” . . . in the generality of states, if every person be left free to have as many children as he pleases, the necessary consequence must be poverty . . .”
But, who will survive? The individuals whose adaptations enable them to survive best in desperate times. Those individuals with those adaptations survive and reproduce and those without them or simply with bad luck die and don’t reproduce and their traits are bred out of the species. If a species is geographically widespread then in the fullness of time members of the species will find themselves in new environments and their optimum adaptations will be new and different and these new individuals will tend to become a new species.
Darwin and Wallace’s key ideas boil down to Malthus.